Internal flaw detection using collimated beams



Oct. 8, 1968 T. H. BRIGGS 3,405,270

' INTERNAL FLAW DETECTION USING COLLIMATED BEAMS 2 Sheets-Sheet. 1

Filed Aug. e, 1965 FROM |NFRARED SOURCE TO INFRARED Lvl/EN TOR A from/EyOct. 8, 1968v T. H. BRIG@ 3,405,270

INTERNAL FLAW DETECTION USING COLLIMATED BEAMS Filed Aug. 6, 1965 2Sheets-Sheet 2 AMPLIFIER 29 ENERGIZING 67 PPLY v 5U SIGNAL READ-OUT 56DEVICE United States Patent Office t 3,405,270I Patented Oct. 8, 19683,405,270 p INTERNAL FLAW DETECTION USING I COLLIMATED BEAMS Thomas H.Briggs, Allentown, Pa., assignor to Western Electric Company,Incorporated, a corporation of New York Filed Aug. 6, 1965, Ser. No.477,710

8 Claims. (Cl. Z50-83.3) f

ABSTRACT F THE DISCLOSURE The presence of internal cracks in a body maybe ascertained by directing, toward av face of the body, a collimatedbeam of Wave energy to whose frequency the body is transparent. Thebodyis rotated about a normal to the face with a constant angularfrequency to cyclically vary the orientation of a crack which may bepresent therein with respect to the incident beam. A'portion of the beamis reflected from the crack with a periodicity determined by the angularfrequency of the rotating body, and is suitably detected.

This invention relates to methods of and apparatus for detectinginternal structural flaws and, more particularly, to methods of andapparatus for detecting internal cracks in silicon bodies. Accordingly,the general objects of the invention are to provide new and improvedmethods and apparatus of such character.

During the processing of polished silicon slices suitable for themanufacture of diffused junction diodes and transistors, a percentage ofthe slices has been found to manifest a brittle condition that generallycausesl subsequent breakage. Such brittleness has been found to becaused by cracks that are formed within Vthe slice prior to or duringpolishing and particularly by those cracks that begin at the polishedsurface and Vextend into the slice.

Past attempts to effectively isolate such cracked slices have beengenerally unsatisfactory. While' certain optical microscopy techniqueshave been employed with limited success to detect surface flaws in suchslices, these techniques have not been found satisfactory for detectinginternal flaws. Even where such internal flaws include portions thatextend to and intersect the surface ofthe slice, such techniques haveproved inadequate to` distinguish the surface portions of such cracksfrom thin surface scratches, which generally are not serious enough tocause failure of the slice.

One reason for the inability of the prior arboptical flaw detectiontechniques to detect internal cracks in silicon, is that thesetechniques employ wave energy of visible light frequencies, to whichsilicon 'slices of practical thickness are Virtually opaque. As aresult, such visible light frequencies do not penetrate far enough intothe slice to yield a reflection of sufficient magnitude to detect acrack if one exists. -It has therefore been necessary to process eitherinadequately inspected slices or, in the alternative, to reject thoseascertainably flawed slices which, if proper inspection techniques wereavailable, might prove to contain only' thin surface scratches.

Accordingly, another object of the invention is to provide new andimproved methods of and apparatus for detecting inter-nal cracks inmaterials opaque to visible light frequencies.

Another object of the invention is'to provide methods of and apparatusfor the simple, rapid and inexpensive, non-destructive detection ofinternal cracks in silicon slices by the use of Wave-energy techniques.

These and other objects of the invention are accomplished by irradiatinga body to be tested for internal cracks with wave energy having afrequency for which the body is essentially transparent, and sensing themaximum level of the energy reflected from the body to ascertain if anycracks are present. The maximum levelof the energy reflected from thebody is of one value if there are one or more internal cracks in thebody, and is of a lesser value if there are no cracks in the body.

In one embodiment of the invention employed to test a silicon slice forinternal cracks, one face of the slice is irradiated by collimatedinfrared waves at a predetermined angle to the face. Advantageously, theslice is rotated about a normal to the face with a constant angularfrequency to` correspondingly vary the orientation of the plane of acrack present therein with respect to the angle of illumination. Aportion of any energy reflected from the slice is converted into anelectrical signal and passed through a transmission-type filter sharplytuned to the angularV frequency of the rotating slice. If a true crackis present in the slice, an electrical signal will appear on anindicator coupled to the output of the filter and tuned to the sameangular frequency.

The nature of the present invention, the manner in which it accomplishesthe above and related objects, and its various advantages and featuresare more fully set forth in the following detailed description ofseveral embodiments thereof, taken in connection with the appendeddrawing, in which:

FIG. 1 illustrates diagrammatically the general principles of theinvention;

FIG. 2 is a perspective view, partly in section and partly in blockdiagram form, of one embodiment of the invention for detecting internalcracks in silicon slices;

FIG. 3 is a fragmentary elevation view of a portion of the embodiment ofFIG. 2; and

FIG. 4 is a fragmentary elevation view of an alternative embodiment ofthe invention.

Referring to FIG. 1, the invention is directed to detecting the presenceof an internal structural flaw, such as a crack, in a body 12.Illustratively, the invention will be described in connection with theinspection of polished monocrystalline silicon slicesused in themanufacture of transistors and diodes. Accordingly, the body 12 willhereinafter be referred to as the silicon slice 12.

It has been determined that an internal structural llaw, such as acrack, within a body presents a directive rellecting surface to Waveenergy penetrating the body and incident upon the crack. The effectivearea of the surface presented to the energy and thus the amount ofenergy reflected, is roughly proportional to the size of the crack andthe orientation of the crack within the body. In the two most widelyused forms of monocrystalline silicon (i.e., those having the so-called1-1-0 and 1-11 crystal structures), cracks tend to occur at angles of60, 45 or respectively, to the polished face of the slice.

Silicon, when utilized in homogeneous slices having a thickness usefulfor diffused junction device manufacture (e.g., 5 to l0 mils), isvirtually opaque to visible light waves but satisfactorily transparentto infrared waves in the 1.5 to 10 micron range.

In accordance with the invention, collimated infrared energy is directedupon a polished face 13 of the silicon slice 12 at a predetermined angleto the face. The inci- -dent energy is refracted at the face 13 whilepassing into the denser medium of the slice 12. If a crack is present inthe slice 12, an effective planar reflecting surface 14 is presentedthereby to the incoming refracted energy. The surface 14 reflects aportion of the incoming energy, and the reflected energy is againrefracted at the face 13 and directed outwardly therefrom to a suitablylocated infrared detector. Generally, the portion of the incomingrefracted energy not reflected by the surface 14, or all of the incidentenergy if there is no crack, passes through the slice 12 and emergesfrom a face 16 opposite face 13.

As will be seen below, the emergent energy is absorbed or dissipated bysuitable means so that none of this energy is reflected back to thedetector.

FIG. 2 depicts a lirst embodiment of the invention utilizing the aboveprinciples for detecting cracks in silicon slices. As shown, a turntable17 is positioned within and concentric to an annular mount 18. Acircumferential track 19 is formed within the mount 18. The turntable 17is mounted for rotation about an axis 21 normal to a mounting face 22 ofthe turntable 17, which is preferably coated with infrared absorbingmaterial (not shown). The turntable 17 is rotated by a drive gear 23that is engageable with a gear 24 mounted at the base of the turntable.A crank 26 is connected to the gear 23 for manually rotating the gear.Alternatively, the gear 24 may be rotated at a constant angularfrequency w1 by a motor 27 carrying .a gear 28 in engagement with thegear 24. The gear 28 can be disengaged from the gear 24 by spring loadedmeans (not shown) of any suitable type.

An angular positioning assembly 29 similar to the type employed in anglemeasuring instruments, such as goniometers, is movably mounted withinthe track 19 of the annular mount 18. The assembly 29 will hereinafterbe referred to as the goniometer 29. The goniometer 29 comprises a pairof curved and slotted brackets 31 and 32, each of which is slidablewithin the track 19. As shown more clearly in FIG. 3, the brackets 31and 32 define concentric circular segments of different mean radii R1and R2. The outer surface 33 of the bracket 31 has a radius slightlysmaller than that of the inner surface 34 of the bracket 32 to preventrotational interference of the brackets 31 and 32 within the track 19.The brackets 31 and 32 are provided at like ends with flattenedextension portions 36 and 37, respectively, slidably mounted within thetrack 19. A surface 38 of the extension portion 36 conforms to and bearsagainst the inner surface 39 of the track 19, and a surface 41 of theextension portion 37 conforms to and bears against the outer surface 42of the track 19. The brackets 31 and 32 are also provided .at theirother ends with overlapping portions 43 and 44, respectively, centeredabout the axis 21. The overlapping portion 44 is provided with aprojecting portion 46 aligned with and rotatable within a bore 47 in theoverlapping portion 43. A crank 48 disposed opposite from and alignedwith the projecting portion 46 extends from the overlapping portion 44to permit manual rotation of the bracket 32 around the axis 21 withrespect to the bracket 31. The brackets 31 and 32 are respectivelyprovided with annular slotted portions 49 and 51 intermediate theirends.

Referring again to FIG. 2, suitable infrared source 52 is aixed, as by apin 53, to the slotted portion 49 of the bracket 31 for irradiating theturntable-mounting face 22. The source 52 is nominally positionedperpendicular to the tangent to the bracket 31 at the mounting point.However, the source may advantageously be pivotally mounted about thepin 53 to permit an added range of adjustment of the angle ofillumination. An optical collimator 54 is mounted in front of andaligned with the source 52 to confine the illumination thereof toessentially a single direction. Electrically, the source 52 is connectedto a suitable energizing supply 56.

An infrared detector 57 is aflxed, as by a pin 58, to the slottedportion 51 of the bracket 32. The detector 57 may be a photocell of thetype that converts infrared energy into an electrical output signal. Thephotocell 57 is nominally positioned perpendicular to the tangent to thebracket 32 at the mounting point; however, the photocell may bepivotally mounted about the pin 58 to provide a limited range ofadjustment. Preferably, an optical collimator 59 is mounted in front ofand in alignment with the photocell 57 in order to focus any incidentinfrared radiation.

The output of the photocell 57 is connected to a contact arm 61 of atwo-position switch 62. A first contact 63 of the switch 62 is connectedto a filter 64 sharply tuned to the angular frequency w1. The output ofthe filter 64 is coupled to an amplifier 66 that is also sharply tunedto the .angular frequency w1. The output of the amplitier 66 is coupledto a suitable indicator device (not shown). A second contact 67 of theswitch 62 s directly connected to a read-out device 68, such as a chartrecorder.

In one method of operation of the above-described embodiment, theunpolished face 16 of the silicon slice 12 to be inspected is allixed tothe infrared absorbing mounting face 22 of the turntable 17 so that theopposite (polished) face 13 of the slice 12 is exposed to the infraredsource 52 and faces the photocell 57. Infrared energy from the source 52is collimated by the lens 54 and is directed upon the polished face 13at a predetermined angle to the axis 21. As shown in FIG. 1, theincident energy is refracted while passing into the denser medium of theslice 12. If `a crack is present in the slice 12, the effectivereflecting plane 14 thereof rellects a portion of the incoming energyroughly in proportion to the size of the crack. Some of the reflectedenergy is refracted at the face 13 toward the photocell 57.

The gear 28 carried by the motor 27 is placed in engagement with thegear 23 of the turntable 17 thereby rotating the turntable at theconstant speed w1. The switch 62 is adjusted so that the contact arm 61registers with the rst contact 63.

Since the turntable 17 is rotated at the constant speed w1 -with respectto the fixed source S2 and the photocell 57, the slice 12 and thereflecting plane 14 also rotate at w1. The planar reflecting surface 14of the crack, being highly directive, thus reflects a maximum amplitudeof infrared energy to the photocell 57 once each lrevolution. As aresult, the photocell 57 generates a pulsating output signal having apulse reptition rate corresponding to the angular frequency w1. Thisoutput signal is transmitted through the filter 64 to be amplied andread out by the amplifier 66.

In order to maximize the amount of reflected energy detected, the crank48 is rotated to locate the optimum circumferential orientation of thephotocell 57 with respect to the source 52. If desired, lthe respectivemounting positions of source 52 and/or photocell 57 may be varied alongthe associated slotted portions 49 and 51.

It has been found that where a large plurality of slices of the sametype of crystal structure are to be inspected, the angles formed bytypical cracks in such slices with respect to their respective polishedsurfaces are statistically determinable within narrow limits. Forexample, in a run of typical silicon samples containing detectablecracks, about percent of the observed cracks were oriented at an angleof between 45 and 60 to the polished surface of the associated slices.Moreover, in practically all of the flawed samples, the cracks wereoriented at angles of between 40 to 70 to the polished surface. Sincethe range of adjustment of illumination and detection angles is smalland relatively predictable in a given production run, a rapid inspectionof any one type of polished slice may be obtained without an undueamount of adjustment.

The above-described arrangement is especially useful in distinguishingsurface scratches from internal cracks that extend to and intersect thesurface. It has Ibeen found that a scratch on the polished surface of aslice under test tends to scatter the wave energy randomly, so that morethan one maximum of reflection occurs during each turntable revolution.In other words, the rellected energy from such a scratch, under theseconditions of rotation, has a pulse repetition rate greater than w1,which corresponds to one maximum per cycle. Since the photocell 57 iscoupled to circuits sharply tuned to w1, no indication of suchscratch-reflected energy will occur.

An alternative way of distinguishing surface scratches from internalcracks is by amplitude discrimination. In view of the energy scatteringnature of surface scratches, the amplitude level of the reflected energyfrom a surface scratch, even at a maximum point, will be less than thatfrom an internal scratch. Accordingly, by placing the contact arm 61 ofthe switch 62 in registration with the second contact `67 and observingthe output of the read-out device 68, surface scratches may bedistinguished from cracks. Thismethod of direct readout of the refiectedinfrared energy may also be used to determine whether there is more thanone crack present in the slice. In this instance, the number ofamplitude maxima observed over a predetermined level, during eachrevolu-tion, indicates the number of cracks in the slice.

Another method of utilizing the above-deseribed` embodiment for crackinspection purposes is to place the motor gear 28 out of engagement withthe turntable gear 24 and to operate the switch 62 so that the contactarm 61 lregisters with the second contact 67. In this case, theturntable 17 is manually positioned by means of the crank 26 until, if acrack is present, a steady maximum response appears on the read-outdevice 68. As described above, further adjustments of the amount ofreflected energy detected may be obtained by (l) rotating the crank 48to vary the orientation o-f the photocell 57 with respect to the source52, and/ or (2) varying the respective mounting positions of thephotocell 57 and/or the source 52 along their associated slottedportions 51 and 49.

FIG. 4 shows an alternative embodiment of the invention which is usefulwhere somewhat less freedom of adjustment of the reflected energy may betolerated. In this embodiment, the source 52 vand the photocell 57 areboth affixed to a slotted portion 69 of a single curved support 71. Asshown, the curved support 71 is mounted on an angle bracket 72 inconcentric relation with a point on the axis 21 adjacent to theturntable 17. The operation of this embodiment is identical to thatdescribed in lconnection with FIGS. 1, 2 and 3, except for thecircumferential adjustment provided by the track 19.

It is to be understood that the above-described embodiments are merelyillustrative of the principles of the invention, and that variousmodifications may be made from the specific details described withoutdeparting from the scope and spirit of the invention.

What is claimed is:

1.l A method of detecting internal cracks in a body, which comprises thesteps of:

directing through the body a collimated beam of wave energy having afrequency for which the body is normally transparent to reiiect aportion of the beam from an internal crack in the body;

imparting relative motion between the beam and the `body to modulate theamplitude of the reflected beam portion when a crack is present in thebody; and

:lensing the amplitude of the beam portion reflected from the bodyduring the relative motion between the beam and the body to ascertainthe presence of a -crack therein.

2. A method according to claim 1 wherein the body is essentiallytransparent to infrared energy and the wave energy has a frequencywithin the infrared spectrum.

3. lA method of detecting internal cracks in a slice that is transparentto and non-absorptive of infrared energy, which comprises the steps of:

directing, toward one face of the slice, a collimated beam of infraredenergy to reflect a portion of the beam from each internal crack in theslice;

rotating the slice about a nonnal to the face; and

detecting, during each revolution, the number and amplitude maxima ofthe beam portions reflected from the slice to ascertain the presence andnumber of cracks in the slice.

4. A method of detecting internal cracks in a slice that 'is transparentto and non-absorptive of infrared energy, which comprises the steps ofdirecting, through the slice, a collimated beam of infrared energy toreflect a portion of the beam from an internal crack in the slice;

rotating the slice with a fixed angular periodicity about a normalthereto for imparting, to the reflected beam portion, a first componenthaving the fixed angular periodicity; and

detecting the first component of the reflected beam portion.

5. A method according to claim 4, in which the slice consists ofmonocrystalline silicon.

6. Apparatus for detecting cracks in a body that is essentiallytransparent to infrared energy, which comprises:

means for supporting the ybody for rotation about a normal to a firstface of the body; a source of infrared energy for rradiating the body ata first angle t-o the normal;

first means movable along a path substantially concentric with a pointon the normal for supporting the source; detecting means for sensinginfrared energy reflected from the body at a second angle to the normal;and

second means movable along a path substantially concentric with the samenormal point for supporting the detecting means.

7. Apparatus as defined in claim 6, wherein the first supporting meansincludes an infrared absorbing surface affixed to the face lof the sliceopposite the first face.

8. Apparatus as defined in claim 6, wherein the detecting means aresensitive only to components of infrared energy occurring at apredetermined angular frequency, and the apparatus further comprisesmeans for rotating the body about the normal at the predeterminedangular frequency.

References Cited UNITED STATES PATENTS 2,318,667 5/ 1943 Bruce Z50-51.5X 2,677,106 4/ 1954 Haynes et al Z50-83.3 X 2,750,512 6/1956 Meloy250--5l.5 2,755,702 7/ 1956 Cook 250-224 X 3,206,603 9/ 1965 MauroZ50-83.3 3,328,000 6/ 1967 Rottmann 250-223 ARCHIE R. BORCHELT, PrimaryExaminer.

